dansmathcast has just crossed over the one billion download milestone! / oh, wait, maybe that was iTunes song downloads / the picture at right is part of the "nested circle world" / the great descartes had a formula back in 1637 for the curvatures of four "kissing circles" / i used it and mathematica to draw hundreds of circles; only a few are shown here / you can learn and see more at my popular website, www.dansmath.com